net: stmmac: socfpga: Remove unused pcs-mdiodev field

net: stmmac: socfpga: Remove unused pcs-mdiodev field

When dwmac-socfpga was converted to using the Lynx PCS (previously
referred to in the driver as the Altera TSE PCS), the
lynx_pcs_create_mdiodev() was used to create the pcs instance.

As this function didn't exist in the early versions of the series, a
local mdiodev object was stored for PCS creation. It was never used, but
still made it into the driver, so remove it.

Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20250424071223.221239-4-maxime.chevallier@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>